The present invention relates generally to expandable retaining rings and, more particularly, relates to a method and apparatus for retaining a propeller shaft housing in a gearcase of an outboard motor.
In order to ensure proper operation of the outboard motor, a propeller shaft housing must be secured within a gearcase. The propeller shaft housing should be secured against rotational, axial, and radial movement. In securing the propeller shaft housing in the gearcase, it is important to maintain a concentric orientation between the gearcase and the propeller shaft housing for proper mechanical operation of the outboard motor.
Outboard motors often incorporate an exhaust passage through the gearcase. This exhaust passage exhausts gases from the outboard motor under the surface of the water so that these gases are not exhausted in the vicinity of persons using the vessel. This method of discharging engine exhaust below the surface of the water also muffles engine noises that are common to all exhaust systems, not only marine systems. It is well known that engine efficiency is a function of exhaust parameters. As such, any unnecessary restriction of the exhaust can negatively affect the efficiency of the engine.
Outboard motors also need to be very durable. These motors can be operated in both salt water and fresh water. Any portion of the motor exposed to these atmospheres needs to be durable and debris and corrosion resistant. In particular, the propeller shaft housing needs to be securely fixed within the gearcase by a means that meets these requirements. The propeller shaft housing is often used to separate the gearcase into two distinct sections. The first section contains the gears of the system. It is generally desirable that this area of the gearcase not be exposed to the exhaust and cooling flows discussed above. The second section of the gearcase is that section that is exposed to the cooling and exhaust flows. Somewhere in this second section is the means of securing the propeller shaft housing in the gearcase. Maintaining the isolation between the first and second sections of the gearcase is important to the operability and life expectancy of the outboard motor. The retainer that secures the propeller shaft housing in the gearcase can help extend the life of the motor in a number of respects, one of which is accurate centering of the propeller shaft in the propeller shaft housing.
Additionally, since outboard motors flow through water, it is most advantageous to provide a smooth hydrodynamic surface to flow therethrough. This flow of water over the gearcase can create unwanted drag on the motor if the housing has protrusions thereon that will decrease the overall efficiency of the motor. A means of securing the propeller shaft housing in/to the gearcase should maintain a minimal profile to minimize hydrodynamic losses associated with the water flow over the gearcase.
There are several known methods of retaining the propeller shaft housing in the gearcase. One such method includes a flange located on a rear facing end of the propeller shaft housing. Fasteners secure the flange of the propeller shaft housing to the gearcase. This method introduces discontinuities on the outer surface of the gearcase which increase hydrodynamic losses. Additionally, the concentricity of the gearcase to the propeller shaft housing is dependent on the mating diameters of the gearcase and the propeller shaft housing.
A second method includes using an externally threaded ring. The threaded ring has a matching thread on an inside surface of the gearcase. The ring is threaded into the gearcase and applies a compression load to the propeller shaft housing thereby seating the housing against a shoulder in the front of the gearcase. This method is prone to damage of the threaded surfaces between the ring and the gearcase during routine maintenance and assembly. Additionally, due to the large diameter of the thread, reasonable tightening torques result in limited and inconsistent compression load on the propeller shaft housing, which, when too low, can lead to radial and rotational movement of the propeller shaft housing within the gearcase.
A third method includes a pair of tabs that are attached to the rear of the propeller shaft housing. These tabs extend beyond an outer diameter of the propeller shaft housing and have an end that is inserted into a corresponding recess in an inside wall of the gearcase. After the propeller shaft housing is positioned in the gearcase, the tabs are inserted into the recesses in the gearcase at a slight angle. As the fasteners that secure the tabs to the propeller shaft housing are tightened, the tabs provide a compressive spring load to the propeller shaft housing thereby securing the propeller shaft housing within the gearcase. The clamp load associated with this method does not totally prevent radial movement at the rear of the propeller shaft housing. This radial movement leads to wear of the mating components of the gearcase and prop shaft housing and can ultimately lead to gearcase assembly failure. A wedge is often disposed between the propeller shaft housing and the inside surface of the gearcase in an effort to prevent wear associated with this radial movement. Implementing this wedge not only increases assembly and production costs but detrimentally affects the concentricity of the propeller shaft housing to the gearcase by biasing the propeller shaft housing away from the wedge and towards an opposite side of the gearcase.
Another method of securing a propeller shaft housing includes positioning a snap ring in a groove in an inside surface of the gearcase in front of the propeller shaft housing. The propeller shaft housing includes a plurality of screws that pass through the front of the propeller shaft housing and into a plate. As the screws are drawn tight, the steel plate is drawn to a front side of the snap ring and the propeller shaft housing is drawn to a shoulder on the inner surface of the gearcase. This method does not secure the rear of the propeller shaft housing and allows for the same type of radial movement of the propeller shaft housing as discussed above, resulting in early failure.
Other methods for retaining a propeller shaft housing within a gearcase are known in the art, but all suffer one or more of the aforementioned shortcomings. It would therefore be desirable to have an apparatus and method capable of securing a propeller shaft housing in a gearcase such that the propeller shaft housing is maintained in a fixed, concentric relationship to the gearcase. It would also be desirable, in securing the propeller shaft housing in the gearcase, to not unduly restrict fluid flow over the gearcase nor restrict engine exhaust flow therethrough.